UPDATE: It appears that the PDF novel and associated DLC may not be available Steam users outside the North America region.
Prior to the release of Metro: Last Light, I was looking through my DLC tab and nothing was there. I restarted Steam today, and while I was looking for something else, this caught my eye:
It isn’t explained to you, but if you navigate to where Steam is installed:
/Steam/steamapps/common/Metro Last Light/novel/
You’ll see the file: Metro 2033 – English text.pdf
Go ahead and copy it out to read on your PC, phone, or other devices. If it didn’t work for you, the DLC can be downloaded (for free), at: http://store.steampowered.com/app/43163/